In the use of high-speed digital circuitry, such as circuitry housing computer signal conditioning equipment manufactured by Cybex Corporation of Huntsville, Ala., the signals originating in such circuitry possess extremely fast rise and fall times, generally in the single-digit nanosecond range, which tends to produce electromagnetic radiation at these frequencies. If not suppressed or blocked, this radiation is capable of producing electromagnetic interference (EMI) with television and radio communications and may further interfere with other digitally operated devices.
In an effort to prevent electromagnetic radiation leakage, manufacturers of digital and other high-speed switching equipment typically resort to enclosing the individual circuit boards which emit the offending radiation inside metallic electromagnetic interference shields, these shields in many instances being mounted and grounded directly to the circuit boards. This is done primarily because the outer enclosure housing the device and its circuit boards may not be constructed of a material suitable for blocking electromagnetic radiation or may be constructed having openings and/or slits that may pass high-frequency noise.
Examples of mounting shielding directly to a circuit board are found in U.S. Pat. Nos. 3,816,911, issued on Jun. 18, 1974, to Knappenberger; 4,386,388, issued on May 31, 1983, to Beun; and 5,014,160, issued on May 7, 1991, to McCoy. These circuit boards with shielding mounted thereto are then mounted in an outer enclosure with other components to form the completed product. In other instances, such is as found in U.S. Pat. Nos. 4,864,077, issued on Sep. 5, 1989, to Wadell; 4,300,016, issued on Nov. 10, 1981, to Bergeron et. al.; 4,661,888, issued on Apr. 28, 1987, to Jewell et. al.; and 5,034,856, issued on Jul. 23, 1991, to Cook et. al., the shielding is constructed in the form of a module within which a discrete circuit board is mounted. Here, the module is then mounted in the outer enclosure with other like-shielded modules to form the completed product.
Problems with these techniques are that the shielding, whether mounted directly to the circuit board or constructed as a module within which the circuit board is mounted, is usually of complex shapes, and in some instances is constructed from a number of asymmetrical sections that must be fitted together. In addition to increasing time required to assemble a device shielded by one of the aforementioned methods of the prior art, fabrication costs are increased due to the assymetrical nature of the discrete sections of the shielding.
Accordingly, since the applicants are unaware of any such outer enclosure that provides efficient EMI shielding, and because the outer enclosure is constructed of symmetrical sections for ease of manufacture, it is an object of this invention to provide an outer enclosure that also serves as an electromagnetic radiation shield by providing an enclosure wherein all joints are of overlapping character.
It is a further object of this invention to provide such an enclosure that is constructed of completely symmetrical components to allow for ease of construction and fabrication.